Which set of three numbers could represent the lengths of the sides of a right triangle?

a.

7, 24, 25

b.

8, 9, 10

c.

9, 11, 14

d.

15, 18, 21

All you have to do is use the Pythagorean theorem (a² + b² = c²) on all of them. Since, the hypotenuse is always the longest side, use the biggest number for c. 
A: 7² + 24² = 25²
   49 + 576 = 625
           625 = 625

Luckily, the answer is A.